opticalstudyon poly(methylmethacrylate)/poly(vinylacetate...

Hindawi Publishing CorporationInternational Journal of PhotoenergyVolume 2009, Article ID 150389, 7 pagesdoi:10.1155/2009/150389

Research Article

Optical Study onPoly(methyl methacrylate)/Poly(vinyl acetate) Blends

R. M. Ahmed

Physics Department, Faculty of Science, Zagazig University, Zagazig 44519, Egypt

Correspondence should be addressed to R. M. Ahmed, rania8 [email protected]

Received 31 March 2009; Revised 26 July 2009; Accepted 26 August 2009

Recommended by Mohamed Sabry Abdel-Mottaleb

Transparent films of poly(methyl methacrylate)/poly(vinyl acetate) blend with different concentrations were prepared by usingsolution-cast technique. FT-IR transmission spectra were carried for the samples to detect the influence of UV radiation.In addition, optical absorption measurements were carried out for the samples at room temperature across the 190–900 nmwavelength regions before and after exposure to UV and filtered radiation using xenon arc lamp. The study has been also extendedto include the changes in the optical parameters including the band tail width and band gap energies for the samples. Moreover,the refractive index was calculated for the samples from specular reflection and absorption spectrum before and after exposure toUV and filtered radiation.

Copyright © 2009 R. M. Ahmed. This is an open access article distributed under the Creative Commons Attribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

The popular and economic method of polymer modificationis blending two or more components with different proper-ties [1]. Polymer blending is an attractive route for producingnew polymeric materials with tailored properties withouthaving to synthesize totally new materials. Other advantagesfor polymer blending are versatility and simplicity [2].Poly(methyl methacrylate) is one of the best organic opticalmaterials and has been widely used to make a variety ofoptical devices, such as optical lenses. It is known thatits refractive index changes upon UV irradiation, eitherin the pure [3, 4] or doped state [5], which provides ameans to fabricate structures, such as gratings or waveguides.On the other hand, poly(vinyl acetate) had the ability toimprove poly(vinyl chloride) photodegradation, photocrosslinking and photo-oxidation by making a blend of them[1]. Also,Kaminska et al. [6] studied the thermal andphotochemical stability of poly(methyl methacrylate)/poly(vinyl acetate) blends and revealed that poly(vinyl acetate)acts as a stabilizer with respect to thermal and photochemicaldegradation when the processes take place in air. Indeed,poly(methyl methacrylate) and poly(vinyl acetate) form animportant pair of polymers.

In the present study, a trail will be carried out to producethe best product of poly(methyl methacrylate)/poly(vinylacetate) blend and also to overcome the defects of theindividual homopolymers. Furthermore, the change in theoptical absorption and the optical parameters will be deter-mined for the samples after exposure to UV (unfiltered light)and filtered radiation. In addition, the refractive index (n)will be calculated for the samples.

2. Materials and Methods

2.1. Materials. Both PMMA (poly methyl methacrylate)and PVAc (poly vinyl acetate) used in this study wereobtained from Sigma, Aldrich (Germany) and were reportedto have molecular weights of 996000 and 167000 g.mol−1 ,respectively. Chloroform has purity 99.8% (HPLC) and wasused as a common solvent for both PMMA and PVAc.

2.2. Preparation of the Samples. Films (thickness 30–40 μm)of (PMMA/PVAc) blends were prepared by using solution-cast technique. Pure PMMA and PVAc were dissolvedseparately in chloroform for 48 hours at room temperature.The solutions of the two homopolymers were then mixed

2 International Journal of Photoenergy

3

6

9

Abs

orba

nce

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber (cm−1)

(a)

3

6

9

Abs

orba

nce

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber (cm−1)

(b)

3

6

9

Abs

orba

nce

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber (cm−1)

(c)

Figure 1: FT-IR spectra of (a) pure PMMA, (b) a blend of (50/50PMMA/PVAc), and (c) pure PVAc before (—) and after exposure toUV radiation (- - -) for 24 hours.

with different concentrations and subsequently cast ontoglass dishes and left in an oven at 40◦C for 24 hours toform transparent films. After curing, the samples of thepure PMMA and PVAc and their blends were removed andthen cut as desired. The concentrations of the prepared(PMMA/PVAc) blends are (0/100, 25/75, 50/50, 75/25,100/0) by weight.

2.3. Absorption Spectra. The absorption spectra wererecorded using Perkin Elmer Lambda 4B spectrophotometer(190–900) nm.

2.4. UV Radiation. The films were exposed to UV andfiltered radiation from a 200 W xenon arc lamp.

2.5. Reflection Measurements. Specular reflection spectrawere recorded with UV-Vis-NIR spectrometer (UV-3101 PC)shimadzu (200–600) nm.

2.6. FT-IR Analysis. FT-IR spectra were measured with aFourier Transform Infrared Spectrometer (FT/IR – 460 plus)in the wave number range (400–4000 cm−1).

3. Results and Discussion

3.1. Characterization of PMMA/PVAc Blend. FT-IR spec-troscopy has long been recognized as a powerful tool for

0

0.5

1

1.5

2

2.5

Abs

orba

nce

200 300 400 500 600 700 800 900

λ (nm)PMMA/PVAc

100/075/2550/50

25/750/100

Figure 2: Absorption spectra of different concentrations ofPMMA/PVAc blend before exposure to radiation.

elucidation of structural information. The position, inten-sity, and shape of vibrational bands are useful in clarifyingconformational and environmental changes of polymers atthe molecular level [7]. FT-IR transmission spectra as shownin Figure 1 have been studied to PMMA, PVAc, and their(50/50 PMMA/PVAc) blend before and after exposure to UVradiation for 24 hours.

It can be observed that a band at 1435.3 cm−1 corre-sponds to asymmetrical bending vibration (CH3) of methylgroup of PMMA. Besides, the vibrational bands observedat 2855 and 1370.7 cm−1 are ascribed to CH3 symmetricstretching and symmetric bending vibrations of pure PVAc,respectively. In addition, a strong band at 1718.3 and1726.9 cm−1 can be attributed to the carbonyl group ofPMMA and PVAc segments, respectively [6]. Also, thefrequency shift of the peak due to C–O band of PMMAaround 1149.9 cm−1 for the sample of the blend implies thatthere is a specific interaction between PMMA and PVAc [7].On the other hand, it can be illustrated that the appearanceof the C–Cl peak characterizing chloroform [8] at 755 cm−1

indicated the presence of solvents molecules.Moreover, the band around 1063.6 cm−1, due to a

stretching vibration of C–O–C group of PMMA, has shiftedto a higher value (1066.9 cm−1) after exposure to UVradiation as well as the methoxy carbon (O–CH3) whichappeared at 2841.6 cm−1. Similarly, the wave numbers cor-responding to the characteristic transmission peaks of PVAcas acetates group at 1370.7 cm−1 have been affected by UVradiation. Also, the decrease in the intensity of carbonylband for PMMA sample (which is more pronounced thanthe intensity decrease of the blend) is an evidence ofside group elimination from PMMA chains upon UVexposure. The broadening of the whole carbonyl band forthe homopolymers indicates that new oxidized groups areformed resulting of photochemical reactions [1, 9]. Besides,the development of the band from 3425 to 3666 cm−1

assigned to OH was observed for pure PMMA and PVAc

International Journal of Photoenergy 3

0

6

12

18

×102

α(μ

m)−

1

300 600 900

λ (nm)

(a)

0

6

12

18

×102

α(μ

m)−

1

300 600 900

λ (nm)

(b)

0

6

12

18

×102

α(μ

m)−

1

300 600 900

λ (nm)

(c)

Figure 3: Absorption spectrum of (a) pure PMMA, (b) blend of(50/50 PMMA/PVAc), and (c) pure PVAc before (—) and after(- - -) exposure to UV radiation for 24 hours.

more pronounced than their blend after UV irradiation. Thischange is an obvious evidence of polymer photooxidation[1]. Consequently, (50/50 PMMA/PVAc) blend has improvedthe degradation of its homopolymer.

Furthermore, the observed decrease of the intensity of IRspectra can be explained on the bases that most syntheticpolymers degrade after exposure to solar ultraviolet (UV)

radiation due to the presence of photosensitive impuritiesand/or abnormal structural moieties which are introducedduring polymerization. The presence of groups such asketones and aldeydes is implicated in polymer degradation[10].

3.2. Optical Absorption Spectroscopy. The study of the opticalabsorption spectra is one of the most productive meth-ods in developing and understanding the structure andenergy gap of amorphous nonmetallic materials. Figure 2shows the absorption spectra of different concentrations of(PMMA/PVAc) blend before exposure to radiation. It can beobserved that the intensity of absorption peak has increasedby increasing the concentration of PMMA in the blend.Moreover, the absorption coefficient (α) has been estimatedfor all samples from (1):

α(υ) = 2.3 log(Ii/It)d

= 2.3Ad

, (1)

where Ii and It are the intensity of the incident andtransmitted light, respectively, A is the absorbance, and d isthe film thickness. Figure 3 shows the absorption coefficientof PMMA, PVAc, and their (50/50 PMMA/PVAc) blendbefore exposure and after exposure to UV radiation. Theobserved wideness of the absorption spectrum of PMMAafter exposure to UV radiation can be attributed to theexistence of more transitions from higher vibration levels ofthe ground state to higher sublevels of the first excited singletstate [11]. In addition, the same behavior observed for PVAccan be interpreted as the existence of ketones and aldeydesdue to the degradation by UV radiation as mentioned before.

3.3. Interband Transitions. When a quantum of radiationis absorbed by a material, the absorption coefficient, as afunction of photon energy for simple parabolic band, can beexpressed by Davis and Mott formula [12], as in (2)

αE = B(E − Eg

)r, (2)

where B is a constant, Eg is the optical band gap of thespecimen, and r is an index having the values of 2, 3, 1/2,and 3/2, depending on the nature of electronic transitionresponsible for the absorption. Figure 4(a) illustrates thedependence of (αE)2 on the photon energy E (eV) for thesamples before exposure to light, which brought in to viewa linear behavior that can be considered as an evidence ofthe direct transition (i.e., for r = 1/2) [13]. The opticalgab was estimated from the intercept on the energy axis ofthe linear fit of the large energy data of the plot [14]. Theabsorption spectra [15] clarify an extending tail for lowerphoton energies below the band edge, which can be describedby (3)

α = αo exp(E

Eu

), (3)

where Eu is the energy of Urbach corresponding to the widthof the band tails of localized states in the band gap. The values


0

40

80

(αE

)2(1

06eV

2cm

−2)

2 3 4 5 6

E (eV)

a

0

20

40

(αE

)2(1

06eV

2cm

−2)

2 3 4 5 6

E (eV)

b

0

40

80

(αE

)2(1

06eV

2cm

−2)

2 3 4 5 6

E (eV)

c

0

40

80

(αE

)2(1

06eV

2cm

−2)

2 3 4 5 6

E (eV)

d

0

30

60

(αE

)2(1

06eV

2cm

−2)

2 3 4 5 6

E (eV)

e

(a)

4

5

6

7

ln(α

)

4 5 6

E (eV)

a

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7

ln(α

)

4 5 6

E (eV)

b

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ln(α

)4 5 6

E (eV)

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7

ln(α

)

5 6

E (eV)

d

5

6

7

ln(α

)

5 5.5 6

E (eV)

e

(b)

Figure 4: (a) The dependence of (αE)2on photon energy E (eV) for (a) pure PMMA, (b) blend of (75/25 PMMA/PVAc), (c) blend of (50/50PMMA/PVAc), (d) blend of (25/75 PMMA/PVAc), and (e) pure PVAc without exposure to radiation.(b) The dependence of (lnα) on photon energy E (eV) for (a) pure PMMA, (b) blend of (75/25 PMMA/PVAc), (c) blend of (50/50PMMA/PVAc), (d) blend of (25/75 PMMA/PVAc), and (e) pure PVAc without exposure to light.

Table 1: The energy gap Eg (eV) for different concentrations of (PMMA/PVAc) blend after exposure to UV and filtered radiation at differentexposure times.

PMMA/PVAc Exposure radiationExposure time (hour)

Percentage of decrease of Eg% (eV)0 4 12 20 24

(100/0)filter 4.60 4.57 4.56 4.52 4.48 2.61

UV 4.60 4.50 4.41 4.32 4.23 8.04

(75/25)filter 4.67 4.66 4.64 4.60 4.59 1.71

UV 4.67 4.54 4.47 4.36 4.30 7.92

(50/50)filter 4.85 4.84 4.83 4.80 4.77 1.65

UV 4.85 4.68 4.57 4.56 4.53 6.29

(25/75)filter 4.92 4.92 4.89 4.88 4.88 0.81

UV 4.92 4.83 4.77 4.73 4.70 4.47

(0/100)filter 5.28 5.28 5.27 5.24 5.24 0.76

UV 5.28 4.25 5.19 5.11 5.07 3.98

of Eu were calculated as the reciprocal gradient of the linearportion of the plot. Moreover, Figure 4(b) shows the plot of(lnα) versus photon energy E (eV) samples before exposureto light.

Tables 1 and 2 summarize the values of optical param-eters (Eg and Eu), respectively, for different concentrationsof (PMMA/PVAc) blend before and after exposure to filteredand UV radiation for 24 hours. It can be deduced thatby increasing the concentration of PMMA in the blend,the values of Eg decreased and the values of Eu increased.Furthermore, Table 1 has illustrated that the decrease of Egvalues by increasing the exposure time of UV radiation was

more obvious than that after exposure to filtered radiationbecause UV radiation has an energy higher than the energyof any bond molecules [16]. Moreover, the blend of (50/50PMMA/PVAc) modified the wideness of PMMA spectra afterexposure to UV radiation.

Also, the values of Eu have increased by increasing theexposure time as indicated in [17]. The increase of Eu valuesby increasing the concentration of PMMA in (PMMA/PVAc)blend can be attributed to the effect of internal potentialfluctuation associated with the structural disorder [11]. Inaddition, the values of constant (B) in (2) were determinedfrom the slope of the linear part of Figure 4(a) for the


Table 2: The band tail width Eu (eV) for different concentrations of(PMMA/PVAc) blends after exposure to UV and filtered radiationat different exposure times.

PMMA/PVAc Exposure radiationExposure Time (Hour)

0 4 12 20 24

(100/0)filter 0.40 0.41 0.43 0.45 0.46

UV 0.40 0.42 0.46 0.49 0.53

(75/25)filter 0.39 0.39 0.43 0.43 0.43

UV 0.39 0.39 0.43 0.45 0.48

(50/50)filter 0.38 0.38 0.41 0.42 0.42

UV 0.38 0.38 0.43 0.45 0.47

(25/75)filter 0.28 0.28 0.30 0.30 0.32

UV 0.28 0.31 0.35 0.38 0.38

(0/100)filter 0.24 0.28 0.30 0.32 0.32

UV 0.24 0.34 0.37 0.37 0.38

Table 3: Values of the constant B (cm−1 eV1/2) for differentconcentrations of PMMA/PVAc blend before and after exposure toUV and filtered radiation for 24 hours.

PMMA/PVAcBefore exposureto radiation

After exposureto filteredradiation

Afterexposure toUV radiation

(100/0) 67.5 58.5 78.8

(75/25) 39.7 78.4 45.5

(50/50) 91.6 78.7 102.6

(25/75) 85.8 69.5 63.2

(0/100) 127.3 123.2 138.5

samples before exposure to light and also after exposure toUV and filtered radiation which are summarized in Table 3.The unit may be given [18] as cm−1 eV(1−r).

3.4. Optical Constants. The absorption coefficient (α) of themedium provides valuable optical information for materialidentification. The attenuation coefficient (k) [19] is directlyproportional to the absorption coefficient (α) as seen in(4)

α = 4πkλ

, (4)

where λ is the free space wavelength of light. For normalincidence, the reflection coefficient (R) [19] is given by (5)

R =[

(n− 1)2 + k2]

[(n + 1)2 + k2

] . (5)

The measurements of specular reflection and theabsorbance are used to calculate the optical constants (n,k) by the two previous equations. Figure 5 illustrates therefractive index values for the blend of (PMMA/PVAc) withvariant concentrations at wavelength of 200 nm. It has beenfound that the value of (n) increases with increasing theconcentration of PMMA in (PMMA/PVAc) blend which isa result of increasing the number of atomic refractions due

1.3

1.35

1.4

1.45

1.5

n

0 20 40 60 80 100

Concentration (wt%)

[Pure PMMA]

[Pure PVAc]

Figure 5: The variation of refractive index (n) versus differentconcentrations of PVAc in (PMMA/PVAc) blends without exposureto radiation at wavelength of 200 nm.

to the increase of the linear polarizability in agreement withLorentz formula [20], as in (6)

n2 − 1n2 + 2

= 43πNαP, (6)

where n, N, and αP are refractive index, the number densityof molecules, and linear polarizability, respectively. On theother hand, Figure 6 shows that there is a minimum peakin the wavelength range of the absorption band followed byan increase of the values of (n), for PMMA, PVAc, and their(50/50 PMMA/PVAc) blend, which clarifies the presence ofnormal dispersion according to Cauchy′s dispersion formula[16], as in (7):

dndλ= −2B1

λ3. (7)

Also, the curves of refractive index (n) show no mini-mum caused by absorption in the visible wavelength, whichis pointing to the fact that all the samples are colorless[21]. In addition, the increase in the values of the refractiveindex after exposure to UV radiation for 24 hours couldbe attributed to localized density increased arising fromphotoinduced cross linking [22]. Moreover, Figure 6 showsthat the increase of refractive index (n) for the samples afterexposure to UV radiation was more obvious than that afterexposure to filtered radiation due to the effect of UV.

4. Conclusions

Blend films of PMMA and PVAc with different concentra-tions have been prepared by casting method, and they wereexposed to UV and filtered radiation for 24 hours. Thefilms were characterized spectroscopically using FTIR whichillustrated that the decrease of the intensity of transmissionspectra of (50/50 PMMA/PVAc) blend after exposure toUV radiation for 24 hours was lower than that of PMMAand PVAc. Consequently, (50/50 PMMA/PVAc) blend hasimproved the degradation of its homopolymer. Furthermore,


1.4

1.6

1.8

2

n

200 400 600

λ (nm)

(a)

1.4

1.6

1.8

2

n

200 400 600

λ (nm)

(b)

1.4

1.6

1.8

2

n

200 400 600

λ (nm)

(c)

Figure 6: Spectral distribution of refractive index (n) for (a) pure PMMA, (b) a blend of (50/50 PMMA/PVAc), and (c) pure PVAc before(—) and after exposure to filtered (- - -) and UV radiation (. . .) for 24 hours.

the calculated values of the optical parameters illustratedthat there was a reduction in Eg values of the films byincreasing the concentration of PMMA in the blend andalso after exposure to UV radiation. Moreover, the refractiveindex values have been calculated from a combinationof reflectance and absorbance measurements at normalincidence for all films which illustrated that there was anincrease of these values by increasing the concentration ofPMMA in the blend and also after exposure to UV radiationfor 24 hours. In conclusion, from all the previous results,it could be concluded that (50/50 PMMA/PVAc) blend hasmodified the optical properties of its homopolymers and alsoit may be suggested to be a good matrix for the dyes used influorescent solar collectors.

References

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[4] A. K. Baker and P. E. Dyer, “Refractive-index modificationof polymethylmethacrylate (PMMA) thin films by KrF-laserirradiation,” Applied Physics A, vol. 57, no. 6, pp. 543–544,1993.

[5] T. Kardinahl and H. Franke, “Photoinduced refractive-indexchanges in fulgide-doped PMMA films,” Applied Physics A, vol.61, no. 1, pp. 23–27, 1995.

[6] A. Kaminska and M. Swiatek, “Thermal and photochemicalstability of poly(methyl methacrylate) and its blends withpoly(vinyl acetate),” Journal of Thermal Analysis, vol. 46, no.5, pp. 1383–1390, 1996.

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[8] M. G. El Shaarawy, A. F. Mansour, S. M. El-Bashir, M.K. El-Mansy, and M. Hammam, “Electrical conduction anddielectric properties of poly(methyl methacrylate)/perylenesolar concentrators,” Journal of Applied Polymer Science, vol.88, no. 3, pp. 793–805, 2003.

[9] H. Kaczmarek and H. Chaberska, “The influence of UV-irradiation and support type on surface properties ofpoly(methyl methacrylate) thin films,” Applied Surface Science,vol. 252, no. 23, pp. 8185–8195, 2006.

[10] H. J. Heller and H. R. Blattman, “Some aspects of stabilizationof polymers against light,” Journal of Pure and AppliedChemistry, vol. 36, no. 1-2, p. 141, 1973.

[11] R. M. Ahmed, Characteristics of some transparent polymerfilms and its application in solar energy conversion, M.S. thesis,Zagazig University, Zagazig, Egypt, March 2004.

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[14] R. Swanepoel, “Determination of surface roughness andoptical constants of inhomogeneous amorphous silicon films,”Journal of Physics E, vol. 17, no. 10, pp. 896–903, 1984.

[15] M. F. Kotkata, H. T. El Shair, M. A. Afifi, and M. M. Abdel Aziz,“Effect of thallium on the optical properties of amorphousGeSe2 and GeSe4 films,” Journal of Physics D, vol. 27, no. 3,pp. 623–627, 1994.

[16] R. M. Ahmed, Studies on fluorescent dye doped in transparentsolid matrix and its application in solar energy collection, Ph.D.thesis, Zagazig University, Zagazig, Egypt, June, 2007.

[17] A. F. Mansour, “Photostability and optical parameters ofcopolymer styrene/MMA as a matrix for the dyes used influorescent solar collectors,” Polymer Testing, vol. 23, no. 3, pp.247–252, 2004.

[18] S. K. J. Al-Ani, Y. Al-Ramadin, M. S. Ahmad, et al., “The opti-cal properties of polymethylmethacrylate polymer dispersedliquid crystals,” Polymer Testing, vol. 18, no. 8, pp. 611–619,1999.

[19] M. Fox, Optical Properties of Solids, Oxford University Press,New York, NY, USA, 2001.


[20] T. Kada, A. Obara, T. Watanabe, et al., “Fabrication of refrac-tive index distributions in polymer using a photochemicalreaction,” Journal of Applied Physics, vol. 87, no. 2, pp. 638–642, 2000.

[21] N. A. Bakr, A. F. Mansour, and M. Hammam, “Opticaland thermal spectroscopic studies of luminescent dye dopedpoly(methyl methacrylate) as solar concentrator,” Journal ofApplied Polymer Science, vol. 74, no. 14, pp. 3316–3323, 1999.

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